1. Field of the Invention
The present invention is generally related to an exhaust system for a marine propulsion device and, more particularly, to an exhaust system which automatically controls the direction of the flow of fluid through an idle relief passage as a function of engine operating speed.
2. Description of the Related Art
Those skilled in the art of marine engines are familiar with various types of idle relief systems which allow exhaust to be emitted from the marine propulsion device above the water level of a body of water when the engine is operating at idle speed but, when the engine is operating at increased speeds, direct the exhaust flow to a position below the level of the water in which the marine propulsion device is operating.
U.S. Pat. No. 3,967,446, which issued to Harralson et al. on Jul. 6, 1976, discloses an exhaust relief silencing apparatus for marine propulsion systems. A tuned exhaust gas relief system includes a lowered driveshaft housing coupled to a two stroke engine by a pair of intermediate stacked exhaust extension plates. The housing directs the exhaust gas downwardly to a through-the-hub exhaust propeller for exit therethrough. With the unit in reverse or idling, exhaust gases are trapped within the housing. A pair of tuned exhaust relief passageways may be formed by cavities in the mating faces of the two extension plates with a pair of inlet openings in the lower wall of the bottom plate.
U.S. Pat. No. 4,668,199 which issued to Freund et al. on May 26, 1987, discloses an idle exhaust relief system for outboard motors. The system includes a main exhaust passageway extending through a partially water filled chamber in the driveshaft housing. An inlet idle relief passage connects the top of the chamber with the main exhaust passageway and an outlet passage connects the top of the chamber with the atmosphere. The system thus defines an effective exhaust silencer for the idle exhaust.
U.S. Pat. No. 4,952,182, which issued to Curtis et al. on Aug. 28, 1990, discloses a noise attenuating exhaust relief system for an outboard motor. It includes an exhaust chamber into which exhaust is discharged from the engine. A first passage in communication with the exhaust chamber provides contraction of the exhaust as the exhaust passes rearwardly, from which the exhaust is discharged into an expansion chamber which substantially surrounds the exhaust chamber. From the expansion chamber, the exhaust is routed through and contracted into a second passage in communication with the expansion chamber, after which it is discharged to the atmosphere. The tortuous path provided by the exhaust relief system, along with the repeated expansion and contraction of the exhaust as it flows to atmosphere, provides a muffling effect at idle operation.
U.S. Pat. No. 5,041,036, which issued to Clark et al. on Aug. 20, 1991, describes an idle exhaust gas relief arrangement for an outboard motor. The outboard motor comprises an internal combustion engine including a lower surface having therein an exhaust gas discharge port, a driveshaft housing having an upper end including an upper face fixed to the lower surface of the internal combustion engine, an outer surface extending downwardly from the upper face, an interior vertically extending main exhaust gas passage extending from the upper face and communicating with the exhaust gas discharge port, an idle exhaust gas relief passage recessed in the upper face and in spaced relation to the main exhaust gas passage, and closed by the lower surface of the internal combustion engine, and an idle exhaust gas outlet port located in the outer surface and communicating with the idle exhaust gas relief passage, and an idle exhaust gas relief tube communicating between the main exhaust gas passage and the idle exhaust gas relief passage. The system has a portion extending vertically within the main exhaust gas passage and terminating in spaced relation above the water level in the driveshaft housing when the driveshaft housing is located in a normal operating position and when the driveshaft housing is at rest relative to the water.
U.S. Pat. No. 5,524,578, which issued to Craft et al. on Jun. 11, 1996, describes a two-cycle engine having an improved idle relief. The engine has an engine block defining at least two cylinders having respective cylinder heads. Pistons are reciprocal within the cylinders. The cylinders have respective fuel inlet ports and exhaust ports and two of the cylinders have an idle relief port disposed between their respective exhaust ports and head ends.
U.S. Pat. No. 6,663,451, which issued to Walczak on Dec. 16, 2003, discloses a siphon pump for a marine propulsion device. A fluid draining device for an outboard motor is provided with a conduit through which exhaust gases are directed. The flow of exhaust gas through the conduit induces a lowered pressure in a central portion of the structure. The reduced pressure magnitude in the central portion of the structure causes a pressure differential in a drain tube that is sufficient to induce a flow of water through the drain tube from a region to be drained toward the central portion. The device uses the Venturi effect to create the lowered pressure. The exhaust gas flow is directly through the conduit from an idle relief exhaust passage to an exhaust port from which the exhaust exits from the marine propulsion system.
The patents described above are hereby expressly incorporated by reference in the description of the present invention.
In certain configurations of outboard motors, the restrictions of certain shapes and configurations of exhaust passages can result in unwanted flow directions of exhaust gases under certain conditions. As an example, the flow restrictions caused by certain shapes and configurations within the exhaust passages can result in excessive exhaust gases passing through an idle relief system of an outboard motor even when the engine is operating at relatively high speeds. This can have an undesirable effect of directing hot exhaust gases into regions of the exhaust passages that can overheat certain components. It would therefore be significantly beneficial if a system could be devised in which the flow through an idle relief system of an outboard motor could be controlled in such a way that exhaust emissions through the idle relief system cease at an appropriate engine speed. It would be further beneficial if this flow through the idle relief system could be reversed to draw ambient air and cause the ambient air to mix with the primary exhaust flow when the engine is operating above idle speed. Lastly, it would be beneficial if a system of this type could be operated without the need for complicated mechanical valving or other moving parts.